Segregation induced core-shell structure

ABSTRACT

A process for synthesis of PtNi high surface area core/shell particles. The processing including formation of PtNi nanoparticles, exposure of the PtNi nanoparticles to oxygen to form a nickel oxide coating on the nanoparticles at the same time the segregation of Ni to surface induces a Pt-skin with PtNi core structure, removal of the nickel oxide coating to form PtNi core/Pt shell (or Pt-skin) structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. Provisional Patent Application No. 62/966,929, filed Jan. 28, 2020, the content of which is incorporated by reference herein in its entirety.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under Contract No. DE-ACO2-06CH11357 awarded by the United States Department of Energy to UChicago Argonne, LLC, operator of Argonne National Laboratory. The government has certain rights in the invention.

TECHNICAL FIELD

The present disclosure relates generally to method of synthesizing and compositions for platinum based nanomaterials, particularly platinum nickel core/shell structures.

BACKGROUND

In the last few decades, significant efforts have been devoted to improving the activity and durability of Pt-based nanomaterials for catalyzing reactions aiming to overcome the high price and scarcity problems of Pt-based catalysts. The use of a bimetallic core/shell (or skin) catalyst has been known (e.g., as described in U.S. Pat. No. 9,246,177). Known methods utilize a multistep approach to achieve the platinum skin on the surface of the nanoparticles by undergoing an acid leach and annealing. The anneal step and its attendant high temperatures result in a number of drawbacks, including sintering of particle and change of shape for nanoparticles with specific morphologies during high temperature annealing. Thus, there is a need for a process to induce a Pt skin on a PtNi nanoparticle without the need for an annealing/sintering.

Embodiments described herein relate generally to oxidative driven segregation induced core/shell structures, including through a process without annealing.

It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the subject matter disclosed herein. In particular, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the subject matter disclosed herein.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing and other features of the present disclosure will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only several implementations in accordance with the disclosure and are, therefore, not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through use of the accompanying drawings.

FIG. 1A is a schematic illustration of a prior art multi-step synthesis procedure of multi-layered Pt-skin nanoparticle catalyst. FIG. 1B illustrates a detailed multi-step process for one prior art process for PtNi synthesis.

FIG. 2 is a schematic illustration of one embodiment of an oxidation induced segregation method for synthesis of PtNi nanoparticles.

FIG. 3A and FIG. 3B illustrate results for experimental embodiments.

FIG. 4A and FIG. 4B illustrate results for experimental embodiments.

FIGS. 5A-5C illustrate results for experimental embodiments.

FIG. 6A illustrates HAADF-STEM results for experimental embodiments. FIG. 6B illustrates BF-STEM results for experimental embodiments.

FIGS. 7A-7C illustrate results for experimental embodiments.

FIGS. 8A-8C illustrate results for experimental embodiments. FIG. 8A illustrates RW084-C-185-1h-A1; Pt:Ni=76.3:23.7. FIG. 8B illustrates RW084-C-185-1h-A1-200; Pt:Ni=75.5:24.5. FIG. 8C illustrates RW084-C-185-1h-A1-400; Pt:Ni=75.1:24.9.

FIGS. 9A-9C illustrate results for experimental embodiments. FIG. 9A illustrates RW084-C-185-20h-A1; Pt:Ni=61:39. FIG. 9B illustrates RW084-C-185-20h-A1-200; Pt:Ni=61:39. FIG. 9C illustrates RW084-C-185-20h-A1-400; Pt:Ni=61.2:38.8.

FIGS. 10A-10C illustrate results for experimental embodiments at 185° C. for 1 hour.

FIGS. 11A-11C illustrate results for experimental embodiments at 185° C. for 20 hours.

FIG. 12A-12D illustrate results for experimental embodiments of RW103-C-1 standard loading. Results are also shown in Table 2 below.

FIGS. 13A-13E illustrate results for experimental embodiments of RW103-C-2 double loading with weight % for Pt at 24.7%. No sintering is observed even for double loading sample. Thus high specific surface area (87.1 m²/gpt) and high specific activity are achieved which guarantee a high mass activity. Results are also shown in Table 3 below.

Reference is made to the accompanying drawings throughout the following detailed description. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative implementations described in the detailed description, drawings, and claims are not meant to be limiting. Other implementations may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and made part of this disclosure.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

One embodiment relates to a synthesis process scalable to industrial production. The synthesis process forms the PtNi nanoparticle having a core/shell (skin) structure. Previous work has reported the formation of such core/shell PtNi nanoparticles through a multi-step process (U.S. Pat. No. 9,246,177) and a one-pot process (U.S. patent application Ser. No. 15/966,297) (both incorporated herein by reference). Both processes utilize the formation of PtNi from metal precursors and require high temperature anneal and acid leaching. FIG. 1A-1B illustrate a prior art multi-step technique utilizing annealing and acid leaching.

In contrast, FIG. 2 illustrates an embodiment of oxidation induced segregated PtNi core/shell nanoparticle synthesis. The method illustrated includes a first step forming PtNi particles, followed by exposure to air at moderate temperature forming a nickel oxide coating on the PtNi particle. The oxygen from air oxidizes the Ni in PtNi nanoparticle to segregate nickel from an outer portion of the coated PtNi particle, forming a Pt rich skin with a PtNi core.

In a first step, the formation of the PtNi may proceed as a multi-step process (for example, as described in U.S. Pat. No. 9,246,177) or a one-pot process (for example, as described in U.S. patent application Ser. No. 15/966,297). The formation of PtNi particles comprises the addition of a nickel precursor, such as nickel acetate tetrahydrate a reducing agent such as 1,2-tetradecanediol, a surfactant such as oleic acid and/or oleylamine, platinum precursors such as platinum(II) acetylacetonate. In one embodiment, a polar solvent such as 1,2-dichlorobenzene is utilized. In one embodiment, the materials are added without order requirement. Prior to addition to the reactor or reaction vessel, one or more of the materials may be purged with an inert gas at greater than room temperature to remove hydrate water, such as from the nickel precursor. The temperature to remove water could be between 100° C. and 150° C. for several minutes to several hours.

The formation of the PtNi particles proceeds by reacting the above noted materials at a reaction temperature, such as between 195° C. and 205° C. (e.g., 200° C.). Reaction time is 10 min to as long as 2 hours.

In a second step, the PtNi particle is reacted with oxygen, for example forming nickel oxide on the surface of the particle. In the oxygen exposure step, the PtNi particle is coated with nickel oxide by reaction via exposure to oxygen. This may be done at a lower temperature, such as 150° C. to 400° C. in air for nanocages and 150° C. to 300° C. in air for nanoparticles. The oxygen exposure time may be, for example, 1 hours to 20 hours, with the time decreasing for as temperature increases. A longer annealing time may be desired in some embodiments, such as up to two days. The amount of nickel oxide that may be formed is limited by the surface nickel content, thus the ratio of Pt:Ni in the nanoparticle impacts the resultant potential oxide formation. In one embodiment, the reaction of nickel and oxygen runs to completion, such that no exposed nickel remains on the PtNi nanoparticles. The PtNi nanoparticles may have a coating of nickel oxide thereon as a result of the reaction, such coating may be solid or may be intermittent on the surface of the nanoparticles.

In a third step, the nickel oxide coating is removed, resulting in a PtNi core/Pt-skin nanoparticle. The nickel oxide coating may be removed by acid leaching. The acid may be, for example, an equal volume of acid (e.g., HClO₄). The material is acid leached for 10 minutes to 24 hours and collected, such as by filtration and washed by water. The collected materials may be dried in a vacuum oven at 80° C. overnight.

The oxidation induced segregation method provides several advantages that are embodied in physical differences between the resultant core/shell nanoparticles of these methods in comparison to prior art leaching/sintering methods.

In one embodiment, the Pt loading is greater than capable under prior methods utilizing an annealing.

The resultant oxidation induced segregated PtNi core/shell nanoparticles exhibit notably higher Pt mass activity.

Examples

The above described results were obtain based upon the following samples.

A protocol for synthesizing core/shell particles having a smooth surface without anneal, as shown in FIGS. 4A and 4B show the results of standard loading and double loading. Table 1 below shows the experimental information.

TABLE 1 SSA 0.9 V 0.95 V 0.9 V 0.95 V (m²/g) (mA/cm²) (mA/cm²) (A/mg) (A/mg) Pt/C TKK 44 0.82 0.12 0.36 0.05 Small scale 40 5 0.85 2 0.34 5 g-Batch 1 60 5.9 0.72 3.5 0.43 5 g-Batch 2 64 6.4 0.73 4.1 0.46 Standard loading 97 5.2 0.58 5.1 0.56 Double loading 87 5.7 0.68 5.0 0.59 The results indicate that higher Pt loading without annealing results in a thinner catalyst layer that provides for lower mass transport resistance in MEA. The process for the experiments proceeded via synthesis of PtNi nanoparticles, loading of PtNi nanoparticles on carbon, coating of nickel oxide layer, removal of nickel oxide layer; and electrochemistry characterization.

Table 2 below shows results for RW103-C-1 standard loading.

TABLE 2 RW103-C-1 standard loading. ECSAHUPD ECSACO CO/ Mass SSA Pt jk0.9 (cm²) (cm²) HUPD (ug) (m²/g) (mA/cm²) 0.9905 1.095 1.11 1.12 97.3 5.23 loading Pt mass jk0.9 jk0.95 jk0.95 ink con on GC loading (A/mg) (mA/cm²) (A/mg) (mg/ml) (ug) (%) 5.09 0.58 0.56 0.50 7.52 15.0

Table 3 below shows results for RW103-C-2 double loading.

TABLE 3 RW103-C-2 double loading. ECSAHUPD ECSACO CO/ Mass SSA Pt jk0.9 (cm²) (cm²) HUPD (ug) (m²/g) (mA/cm²) 0.7635 0.8046 1.05 0.92 87.1 5.74 loading Pt mass jk0.9 jk0.95 jk0.95 ink con on GC loading (A/mg) (mA/cm²) (A/mg) (mg/ml) (ug) (%) 5.0 0.68 0.59 0.25 3.74 24.7

Definitions

As used herein, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, the term “a member” is intended to mean a single member or a combination of members, “a material” is intended to mean one or more materials, or a combination thereof.

As used herein, the terms “about” and “approximately” generally mean plus or minus 10% of the stated value. For example, about 0.5 would include 0.45 and 0.55, about 10 would include 9 to 11, about 1000 would include 900 to 1100.

It should be noted that the term “exemplary” as used herein to describe various embodiments is intended to indicate that such embodiments are possible examples, representations, and/or illustrations of possible embodiments (and such term is not intended to connote that such embodiments are necessarily extraordinary or superlative examples).

The terms “coupled,” “connected,” and the like as used herein mean the joining of two members directly or indirectly to one another. Such joining may be stationary (e.g., permanent) or moveable (e.g., removable or releasable). Such joining may be achieved with the two members or the two members and any additional intermediate members being integrally formed as a single unitary body with one another or with the two members or the two members and any additional intermediate members being attached to one another.

It is important to note that the construction and arrangement of the various exemplary embodiments are illustrative only. Although only a few embodiments have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter described herein. Other substitutions, modifications, changes and omissions may also be made in the design, operating conditions and arrangement of the various exemplary embodiments without departing from the scope of the present invention.

While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any inventions or of what may be claimed, but rather as descriptions of features specific to particular implementations of particular inventions. Certain features described in this specification in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 

We claim:
 1. A method of synthesizing Pt shell PtNi core nanoparticle comprising the steps of: forming a reaction mixture comprising a nickel precursor, a reducing agent, and a surfactant comprising one part oleic acid to two parts oleylamine, a platinum precursor, and a polar solvent; reacting the reaction mixture at between 195° C. and 205° C. for at least 10 minutes forming a PtNi nanoparticle solution; isolating PtNi nanoparticles from the PtNI nanoparticle solution; and exposing the PtNi nanoparticles to oxygen; forming nickel oxide by reaction of nickel of the PtNi nanoparticles with the oxygen; removing the nickel oxide from at a portion of the PtNi nanoparticles.
 2. The method of claim 1, wherein isolation of the PtNi nanoparticles comprises sonicating the PtNi nanoparticle solution with substrate in chloroform solution, adding hexane to the sonicated chloroform solution, precipitating PtNi nanoparticles, and collecting the PtNi nanoparticles by filtration.
 3. The method of claim 1, wherein exposing the PtNi nanoparticles comprises exposure at a temperature of 150° C. to 400° C.
 4. The method of claim 3, wherein exposing the PtNi nanoparticles comprises exposure at a temperature of 150° C. to 300° C.
 5. The method of claim 3, wherein the exposing the PtNi nanoparticles is for 1 to 20 hours.
 6. The method of claim 1, wherein removing the nickel oxide is by acid leaching.
 7. The method of claim 6, wherein the acid leaching is by exposure of an equal volume of acid to nanoparticle.
 8. The method of claim 6, wherein the acid leaching is by exposure to HClO₄
 9. A method of synthesizing Pt shell PtNi core nanoparticle comprising the steps of: forming a reaction mixture comprising a nickel precursor, a reducing agent, and a surfactant comprising one part oleic acid to two parts oleylamine, a platinum precursor, and a polar solvent; reacting the reaction mixture at between 195° C. and 205° C. for at least 10 minutes forming a PtNi nanoparticle solution; isolating PtNi nanoparticles from the PtNI nanoparticle solution; and exposing the PtNi nanoparticles to oxygen at a temperature of 150° C. to 400° C. for 1 to 20 hours; forming nickel oxide on a surface of the PtNi nanoparticles by reaction of nickel of the PtNi nanoparticles with the oxygen; removing the nickel oxide from at a portion of the PtNi nanoparticles.
 10. The method of claim 9, wherein isolation of the PtNi nanoparticles comprises sonicating the PtNi nanoparticle solution with substrate in chloroform solution, adding hexane to the sonicated chloroform solution, precipitating PtNi nanoparticles, and collecting the PtNi nanoparticles by filtration.
 11. The method of claim 9, wherein exposing the PtNi nanoparticles comprises exposure at a temperature of 150° C. to 300° C.
 12. The method of claim 9, wherein removing the nickel oxide is by acid leaching.
 13. The method of claim 12, wherein the acid leaching is by exposure of an equal volume of acid to nanoparticle.
 14. The method of claim 12, wherein the acid leaching is by exposure to HClO₄. 